It also pertains to an integrated circuit, in particular an analog/digital converter, provided with such a system.
Certain signal processing functions used in analog integrated circuits are based on the value of a time constant RC. Typically, the coefficients of an analog filter or else of an analog digital converter, such as a continuous-time delta-sigma analog digital converter, are achieved by means of resistance R and capacitances C.
The product RC of an analog filter corresponds to its cutoff frequency. For a continuous-time delta-sigma analog digital converter (ADC), this time constant RC is related to its sampling frequency. In both these cases, the precision of the time constant RC is important. For the cutoff frequency, if RC is estimated with too high a value, the filter cuts off a part of the signal, and if RC is too small, then the filter does not attenuate the signal sufficiently. In the case of the delta-sigma modulator, it is the performance and the stability of the modulator that depend directly on the precision of the value of RC. The RC coefficients achieve the transfer function of the continuous-time delta-sigma ADC.
Depending on the application, the precision with which the product RC must be known may be high or low. A precision of the value of the time constant RC of +/−5% is generally sufficient.
However, an integrated resistance is accuracy of +/−15% while the value of an integrated capacitance varies at least by +/−20% depending on the technology. In conclusion, the time constant RC that has a precision of +/−35%, is not sufficient for a great majority of applications.
Various schemes for calibrating a time constant are already known in the prior art. Reference may be made in this regard to the documents U.S. Pat. Nos. 6,169,446, 6,262,603, 6,803,813 and 7,078,961.
The calibration described in these documents is based on a comparison of the value of the time constant defined by the product RC with a known and accurate time base formulated, for example, by a quartz-driven clock, or on a comparison of the voltage across the terminals of the resistance R with that present across the terminals of the capacitance C when they are traversed by the same current.
These schemes for calibrating the time constant RC merely deliver a digital word, which directly controls a resistance or a variable capacitance.
Moreover, they are all sensitive to the analog imperfections intrinsic to the hardware components used to make the converter, such as the offset voltage of an operational amplifier or of the comparators used.